Problem: The geometric sequence $(a_i)$ is defined by the formula: $a_1 = -1$ $a_i = 2a_{i-1}$ What is $a_{3}$, the third term in the sequence?
Solution: From the given formula, we can see that the first term of the sequence is $-1$ and the common ratio is $2$ To find the third term, we can rewrite the given recurrence as an explicit formula. The general form for a geometric sequence is $a_i = a_1 r^{i - 1}$ . In this case, we have $a_i = -1 \left(2\right)^{i - 1}$ To find $a_{3}$ , we can simply substitute $i = 3$ into the formula. Therefore, the third term is equal to $a_{3} = -1 \left(2\right)^{3 - 1} = -4$.